1. Technical Field of the Invention
The present invention relates generally to the mobile telecommunications field and, in particular, to a method for processing multiple random access mobile-originated calls in rapidly varying radio channels.
2. Description of Related Art
The next (so-called xe2x80x9cthirdxe2x80x9d) generation of mobile communications systems will be required to provide a broad selection of telecommunications services including digital voice, video and data in packet and channel circuit-switched modes. As a result, the number of calls being made is expected to increase significantly, which will result in much higher traffic density on random access channels (RACHs). Unfortunately, this higher traffic density will also result in increased collisions and access failures. Consequently, the ability to support faster and more efficient random access is a key requirement in the development of the new generation of mobile communications systems. In other words, the new generation systems will have to use much faster and more flexible random access procedures, in order to increase their access success rates and reduce their access request processing times.
In a Direct Sequence-Code Division Multiple Access (DS-CDMA) mobile communications system, in order for a mobile station to commence radio communications with a base station, radio channel resources are allocated (dedicated) to the connection (for both mobile-originated and mobile-terminated calls). Typically, the mobile station contacts the base station on a RACH, which is a common (shared) channel. As such, requests to set up a dedicated connection are often transmitted on a RACH. Alternatively, short packets of user data can be transmitted on the RACH as well.
In many mobile communications systems, a slotted-ALOHA (S-ALOHA) random access scheme is used. For example, systems operating in accordance with the IS-95 Standard (ANSI J-STD008) use an S-ALOHA random access scheme. Typically, an S-ALOHA random access scheme is employed to enable several MSs to use the same physical channel. For example, using a basic S-ALOHA random access scheme, there are well-defined instants in time (time slots) at which random access transmissions are allowed to begin. A mobile station randomly selects such a time slot for transmission of a random access burst. The mobile station then listens to a downlink common physical channel for an acknowledgment from the base station that the random access burst was received. However, these time slots are not pre-allocated to specific users"" mobile stations. Consequently, collisions between the different mobile stations"" random access bursts can occur. As such, if one mobile station""s RACH burst collides with another mobile station""s RACH burst, this collision problem is resolved by having the mobile stations re-transmit their RACH messages in a respective allowed time slot (after a random waiting time). However, if the traffic load is relatively high, this collision resolution approach can be inadequate and collisions can still occur.
A number of other random access collision resolution approaches have been proposed. For example, in a specific mobile communications system using an S-ALOHA random access scheme, such as the method disclosed in the above-cited U.S. patent application Ser. No. 08/733,501 (hereinafter, xe2x80x9cthe ""501 Applicationxe2x80x9d), a mobile station generates and transmits a random access packet. A diagram that illustrates a frame structure for such a random access packet is shown in FIG. 1. The random access packet (xe2x80x9caccess request data framexe2x80x9d) or xe2x80x9cburstxe2x80x9d comprises a preamble (10) and a data or message part (12). Typically, the preamble does not include user information and is used in the base station receiver primarily to facilitate detection of the presence of the random access burst and derive certain timing information (e.g., different transmission path delays). Note that, as illustrated in FIG. 1, there can be an idle period (14) between the preamble and message part during which time there is no transmission.
In order to reduce the risk of collisions between the random access bursts of two mobile stations that have selected the same time slot, the concept of burst xe2x80x9csignaturesxe2x80x9d has been introduced. For example, as described in the ""501 Application (see FIG. 2), the preamble of a random access burst is modulated with a unique signature pattern. Also, the message part is spread with a code associated with the signature pattern used. The signature pattern is randomly selected from a set (e.g., one or more) of patterns that can be, but are not necessarily, orthogonal to each other. Since a collision can occur only between mobile stations"" bursts that are using the same signature, the risk of a random access collision is reduced in comparison with other existing schemes. As such, the use of this unique signature pattern feature, as described and claimed in the ""501 Application, provides a significantly higher throughput efficiency than prior random access schemes.
The above-cited U.S. patent application Ser. No. 09/169,731 (hereinafter, xe2x80x9cthe ""731 Applicationxe2x80x9d) describes and claims a novel format for an uplink common physical channel in a random access mobile communications system, in which a mobile station transmits a predetermined signature pattern in parallel with the message or data part of the random access request. Consequently, the signature portion of the random access request can also function as a pilot by providing additional energy for channel estimation during the data part of the request, while advantageously reducing the amount of overhead signalling involved. This additional energy is especially useful in attempting to ensure sufficiently high quality coherent detection of the data portion in a rapidly varying radio channel environment.
Nevertheless, ideally in a rapidly varying radio channel, the energy used for channel estimation should be spread out in time over the data or message part, in order to achieve a radio channel estimate of sufficient quality during that portion of the random access request. Even if a channel estimate of sufficient quality can be achieved during the preamble (due to the distinctive signature in the preamble), in a rapidly varying channel, this estimate might not be valid for a significant part of the data portion of the random access request. As such, it is important to provide enough energy in the preamble for the receiver to detect the preamble and correctly identify the channel paths.
On the other hand, in a rapidly varying radio channel, it is also important to provide enough energy in the pilot to ensure proper coherent detection of the data portion. Unfortunately, these two important but conflicting energy requirements in an uplink common physical channel format result in the transmission of random access requests with excessive overhead signalling. In other words, the ratio of xe2x80x9coverheadxe2x80x9d energy (preamble+pilot energy) to the xe2x80x9cdataxe2x80x9d energy is unnecessarily high, with its attendant disadvantages.
Notably, although the above-described random access methods and solutions have numerous advantages over prior random access schemes, a number of problems still exist that remain to be solved. For example, regardless of whether or not a signature pattern is transmitted in a preamble or in parallel with a preamble, it can be assumed that the transmitted signature pattern will be detected at a base transceiver station using coherent correlation detection techniques. However, in that regard, because of the doppler spread of the radio channel used (due to the mobile station""s motion) as well as the various frequency errors that can exist between the mobile station and base station, the radio channel can vary so rapidly during the signature""s duration that coherent correlation cannot be accomplished at the base station""s receiver. This problem is particularly pronounced for the higher carrier frequencies, because the maximum doppler spread of a radio channel is proportional to the carrier frequency being used. Nevertheless, as described in detail below, the present invention successfully resolves these and other related problems.
In accordance with a preferred embodiment of the present invention, a method for improving the performance of a random access communications system in a variable radio environment is provided whereby at least one valid set of burst signatures is used for transmission by one or more mobile stations. Each set includes at least one signature with a different signature-length than the signatures in other sets. The different signature-lengths can be optimized for the operational environments involved. For example, a set of relatively long signatures can be used for slower-moving mobile stations, such as in an indoor cell. A set of relatively short signatures can be used for high-speed mobile stations, such as in cells covering a highway. Depending on the operational environment, the signature-lengths in the other sets can be somewhere in between the lengths of the short and long sets. Also, in accordance with a second embodiment of the present invention, at least one differentially-encoded signature is used for random access transmissions, in order to reduce the radio channel""s sensitivity to large doppler spreads and frequency errors.
An important technical advantage of the present invention is that the random access approach used will be less sensitive to fast-fading and frequency errors than prior random access approaches.
Another important technical advantage of the present invention is that the performance of the random access approach used will be significantly higher than prior random access approaches.